1. Field of the Invention
The present invention relates to a method of forming a thin film on a semiconductor wafer, and more specifically, to a method of forming a thin film with a flat surface on a semiconductor wafer.
2. Description of the Prior Art
Silicon oxide, having a proper dielectric constant and excellent cohesion to silicon surfaces is commonly used in gate oxides, local oxidation of silicon (LOCOS), field oxides, interlayer dielectrics, pad oxides, etc. As the integration density of the electrical devices fabricated on the wafer increases, the standard for the quality and the step coverage ability of the silicon oxide becomes more rigid.
There are three commonly adopted methods for forming a silicon oxide film on a surface of a semiconductor wafer: (1) chemical vapor deposition (CVD), (2) thermal oxidation, and (3) spin-on glass (SOG). The CVD processes include silane low-pressure chemical vapor deposition (SiH4-LPCVD), tetra-ethyl-ortho-silicate low-pressure chemical vapor deposition (TEOS-LPCVD), plasma-enhanced chemical vapor deposition (PECVD), etc. Generally a silicon oxide film formed by the CVD process has better step coverage ability. The CVD process involves changing reactants to solid products through chemical reactions in a chamber, and is one of the most important tools commonly applied in the semiconductor industry. Since the CVD process involves producing films through chemical reactions among gas reactants, the crystallinity and the stoichiometry of the films are better than that of the films produced by sputtering.
However, the thickness uniformity of the thin film, especially thin films composed of large molecule precursors, formed by the PECVD process is easily affected by the operating temperature of the PECVD process so as to reduce yield rates in subsequent processes. This negative effect is normally compensated by adjusting parameters of the PECVD process, including gas flow rate, pressure or showerhead spacing. However, such adjustments normally change the film property and narrow the process window. As specifications for the semiconductor devices become more and more complicated, the requirement for the thickness uniformity of the thin film tends to be more and more rigid as well. Thus it is important to improve the thickness uniformity of the thin film without changing the film property.
It is therefore a primary object of the present invention to provide a method of using a precursor A to form a thin film on a semiconductor wafer so as to improve the thickness uniformity of the thin film.
According to the claimed invention, a surface of a semiconductor wafer has at least a first region, containing an inner portion of the wafer, and a second region, containing an outer portion of the wafer, and slopes outward from the first region to the second region. An in-situ inert gas plasma treatment is performed on the surface of the semiconductor wafer to generate different temperatures from the first region to the second region. Different deposition rates of the precursor A from the first region to the second region are thus formed so as to form the flat surface. A precursor A-chemical vapor deposition (CVD) process is performed immediately after performing the inert gas plasma treatment so as to form the thin film with the flat surface.
It is an advantage of the present invention against the prior art that a temperature gradient is formed from the first region to the second region so as to generate different deposition rates of the precursor A from the first region to the second region. The defective thickness uniformity of the thin film is thus improved without changing the film property. Consequently, the yield rates of subsequent processes and the reliability of the product are both significantly improved without narrowing the process window.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the figures and drawings.